The present invention relates to the control of printing heads, particularly for impact printers such as dot matrix printers operating at very high data rates. In particular, this invention relates to control of bounce of the printing element on the backstop at the end of a printing cycle.
This invention applies to printers which develop a force whose direction is a function of the direction of the driving pulse. Generally, these involve a plurality of printing elements, each including a moving electromagnetic coil in a magnetic field, having attached thereto a print wire or stylus, the styli being arranged in one or more vertical lines and being maintained in a spaced-apart arrangement in a print head. One such print head is disclosed in U.S. Pat. No. 4,129,390, granted to J. E. Bigelow et al on Dec. 12, 1978, and comprises a stacked array of flat blade type printing elements.
Each printing element or blade has associated therewith a drive circuit for controlling the operation thereof. Signals from an associated source such as a keyboard are fed to a matrix encoder which converts them to signals of the matrix format for controlling the print wire drive circuits. The drive circuits usually tend to supply constant energy drive pulses, based upon the assumption that constant energy drive pulses always produce constant impact forces and uniform print density.
Each printing element is normally biased against a backstop and, when a drive pulse is applied to the printing element, it is driven from the backstop toward a record medium for printing indicia, the printing element then rebounds to the backstop. Ideally, the backstop would absorb all of the kinetic energy of the printing element and stop it dead still. Practically speaking, however, backstops are incapable of performing this way. Thus, the printing element will rebound from the backstop and move along a path back toward the record medium. If the energy remaining in the printing element after backstop rebound is sufficient, the printing element may rebound all of the way to the record medium and cause a spurious dot to be printed. Even if the rebound is only part way to the record medium, the printing element, being displaced from its normal rest position, is not positioned to respond favorably or accurately to a subsequent drive pulse, nor will it be until it reaches quiescence at or near the backstop. Thus, there is an inherent delay and significant reduction in maximum printing speed, since the minimum time interval between succeeding drive pulses applied to a printing element must be at least as great as the time required for the printing element to move from the backstop to the printing medium and back again.
Several attempts have been made in the past to overcome the problem of bounce of the printing element. U.S. Pat. No. 3,172,353, which issued to C. J. Helms on Mar. 9, 1965, discloses means for extending the length of the drive pulses beyond the time at which a print hammer strikes the record medium so that the print head magnetic field will oppose the print hammer rebound and thereby dissipate a portion of the rebound energy so as to minimize backstop wear. This type of system presumes an accurately fixed travel distance of the printing element between the backstop and the record medium, since the ratio of the accelerating portion of this drive pulse to the returning portion thereof is fixed.
In many matrix printers, on the other hand, the travel distance is variable within relatively wide limits, depending upon the wear of the backstop, thickness of the paper, etc. Furthermore, by applying braking current during the printing dwell time that the printing element is in engagement with the record medium and during a portion of the return flight of the printing element to the backstop serves to increase the dwell on the record medium and to reduce the return velocity toward the backstop. Thus, the round trip time for the printing element is significantly increased, thereby reducing the speed capability of the printer. In this regard, it is significant that the prior art patent relates to a line printer wherein each print hammer is located at a given column location and prints a complete character. In such printers, the repetitive capability of the printing element is not nearly so important as in a matrix printer since such elements may operate only once for each line of print when printing fully formed characters. Finally, by extending the drive pulse through and beyond the point of impact of the printing element with the record medium, prior art systems expend significant amounts of additional energy and always adds to the heat that must be dissipated by the hammer drive.
U.S. Pat. No. 3,678,847, which issued July 25, 1972 to C. B. Pear, Jr. et al, discloses a circuit for applying a damping pulse to the printing element during its return flight to the backstop. This system, like the earlier mentioned approach, increases the round trip travel time of the print hammer and increases heat energy accordingly.